1. Field of the Invention
The present invention relates to an oxygen concentration detector which can accurately and rapidly detect particularly an oxygen concentration in a low temperature gas.
2. Description of the Prior Art
Heretofore, the oxygen concentration detector which determines an oxygen concentration in combustion gas exhausted from an internal combustion engine based on the principle of an oxygen concentration cell by using oxygen ion conductive solid electrolyte, has been widely known. For example, the oxygen concentration detector using zirconia ceramics added with yttria as a solid electrolyte and platinum as electrodes has been mainly practically used. This oxygen concentration sensing device is used for detecting a point where an air-fuel ratio .lambda. of the internal combustion engine is 1.0 but in order to detect the point where .lambda. is not 1.0 based on Nernst equation, it is necessary not only to accurately determine an electromotive force of the oxygen concentration cell but also to accurately determine the temperature. For the purpose, for example, it has been proposed that as shown in FIG. 1, a temperature detecting element 2 is inserted and disposed in a cylinder of a tubular solid electrolyte 1, one end of which is closed and which is provided with electrodes 3 and 4 on the inner and external surfaces and the temperature of the solid electrolyte 1 is determined. However, the temperature of the solid electrolyte when being exposed to the exhaust gas, is not uniform throughout the whole but causes unevenness owing to the flow rate of the exhaust gas, so that it is impossible to show the whole solid electrolyte by the measurement at only one point. Furthermore, when the exhaust gas temperature is varied, a time delay occurs and it is very difficult to accurately determine the temperature of the solid electrolyte and the structure becomes complicated.
In such an oxygen concentration detector, at a low temperature, the catalytic activity of platinum and the like lowers and the electric resistance of the solid electrolyte itself become high and the impedance as the oxygen detector becomes higher, the influence of noise is high and the response is slow, so that the lower limit of the practically used temperature is about 350.degree. C. However, the temperature of the exhaust gas of the internal combustion engine may be lower than said lower limit when starting or driving at a low speed and it is impossible to fully develop the function. For obviating this defect, it has been proposed to insert a coil-formed heating wire 5 in a cylinder of the solid electrolyte 1 as shown in FIG. 2 to heat the solid electrolyte 1 but in this case the structure becomes complicated and the electric power as high as several tens watts is required. In addition, it has been proposed that as shown in FIG. 3, a temperature detecting element 2 and a coil-formed heating wire 5 are inserted in a cylinder of the solid electrolyte 1 to heat the solid electrolyte 1 and concurrently detect the temperature but the structure in this case becomes more complicated and this device is poor in the practicability.